This invention relates to solar stills for producing fresh water, in other words domestic equipment or industrial plant units employing solar energy for producing demineralized water from natural non-potable water. Where this non-potable water is sea water, such stills could also, as a useful by-product, produce brine.
The problems, both economic as well as technical, that sea water desalination installations of different types (distillation and filtration) pose, have been set out in detail in an article by Andy Coghlan, entitled xe2x80x9cFresh water from the seaxe2x80x9d, published on pages 37 to 40 of the British journal New Scientist of Aug. 31, 1991. In this article, we are reminded how vital it is becoming to rapidly develop techniques which are both effective and inexpensive for desalinating sea water, in order to meet the exponential requirements for fresh water in dry zones of the earth.
To meet this problem, numerous solutions for distilling sea water have been proposed which call upon the relatively intense and free energy from the sun, as a replacement for the costly energy produced by fossil fuels. The solutions have been the subject matter of numerous patents and articles from which two are of special interest, and are taken as references in view of their particular relevance.
European Patent 0, 612,691 published in 1994, filed by Mitsubishi, discloses a solar unit for producing fresh water of the conventional type, in which a reservoir having a black base, containing sea water, is installed beneath a transparent space having a sloping roof. Two gutters, intended to recover the fresh water which runs down the vertical walls of the space, are arranged at the foot of these walls. In order to prevent drops of fresh water, condensed on the inner space of the roof and which, moreover, reflect a portion of the solar radiation, from falling back into the reservoir, this face is provided with a wettable covering, which is transparent or at least translucent, which prevents water droplets from forming. Additionally, in order to decrease the temperature of the roof of the space and thus improve vapor condensation, sea water is constantly spread over this roof. In this way, the yield of such an installation is improved. At least two disadvantages nevertheless remain: (1) an excessive amount of water is constantly heated by the sun, which decreases the maximum temperature of the mass of water in the reservoir and reduces the degree of evaporation achieved and (2) all the latent heat of the water vapor condensation is lost.
The article from New Scientist cited above carries, on page 39, a brief description of a piece of domestic equipment for producing fresh water which is original, has a high yield, and employs solar energy. This equipment, developed by P. Le Goff, carries, below a transparent plastic membrane, an orientable mirror that reflects the solar energy towards the front black face of a first aluminium plate, arranged vertically. The rear face of this first plate is coated with a gauze, which is supplied with sea water under gravity. Several identical plates provided with this same hydrophilic covering are arranged in cascade a few centimeters from each other. The solar energy heats the first plate up to a about 94xc2x0 C., the effect of which is to evaporate a relatively large amount of the water circulating in the lining covering the rear face. The water vapor thus produced in the space separating the first and second plates condenses on the front face of this second plate; the effect of this is to cause it to heat up, leading, in its turn, to evaporation of a large amount of the salt water circulating on its rear face. This goes on up to the sixth plate, which gets heated up to 45xc2x0 C. The fresh water condensed on the front face of each plate starting from the second one, is collected by a collector. Another collector, which is not shown, collects the brine that appears at the bottom of each lining. The manufacturer announces a daily production of 20 liters of fresh water per square meter of a plate exposed to the sun. Such a high yield is the result of recovery, on the second to sixth plates, of the latent heat of condensation of the water vapor produced by the hot linings, and which are kept constantly humidified, of the first to fifth plates. According to the author of the article, this result compares very advantageously with results (2 to 3 liters a day and per square meter) from sea water solar distillation equipment of the conventional type. This apparatus is effective and suitable for domestic equipment but it is relatively expensive to construct due to some of the components (the heliostat and the aluminium plates) that it includes.
In the text that follows, we shall take the yield of a solar still for producing fresh water to mean the ratio between, on the one hand, the amount of fresh water effectively produced per hour of average sunlight and per square meter of surface absorbing the solar radiation and, on the other hand, the amount of water Q, theoretically evaporated by the heat of the sun, absorbed by this unit of surface area during this unit of time (giving Q=1.5 kg/h.m2 for an average level of sunshine of one kilowatt/m21, in dry zones).
The first aim of the invention is to construct improved solar stills for producing fresh water.
The second aim of the invention is to construct such stills having a yield that is as high as possible, while simultaneously requiring a relatively low initial investment and operating and maintenance costs that are particularly low.
The third aim of the invention is to provide solar stills for producing fresh water in which as much as possible of the latent heat of condensation of the vapor is recovered.
The fourth aim of the invention is to provide such solar stills which are readily adaptable to the particular conditions of their implementation.
The fifth aim of the invention is to provide such solar stills for producing fresh water, that are well suited to withstand strong winds.
The sixth aim of the invention concerns industrial solar plant producing fresh water by distillation of sea water and comprising a relatively large number of identical solar stills.
The seventh aim of the invention concerns industrial plant that combines a solar plant for producing fresh water by distillation of sea water and a salt marsh supplied by the brine furnished by this plant.
The eighth aim of the invention concerns a novel composite product, specially adapted to the construction of improved solar stills for producing fresh water according to the invention.
According to the broadest formulation of the invention, there is provided a solar still for producing fresh water comprising:
a device, adapted to absorb solar radiation and to contain water to be distilled;
a condensation surface on which vapor, produced by heating up of water of the device, can condense;
means for collecting the fresh water that trickles down the condensation surface;
characterised in that:
said device comprises an impermeable membrane, that is flexible, kept stretched, of a dark color, exposed to the sun and, arranged in the shade and forming a covering for said membrane, a hydrophilic fleece, supplied with water by capillarity and gravity;
means for recovering brine are arranged at the bottom of said fleece.
It will be immediately noted that the more or less hydrophilic nature of a material is measured by the degrees of capillarity that it has vis-a-vis liquids able to wet it. Under these conditions, a wettable but only slightly hydrophilic fleece will be supplied by gravity, by pouring the water to be distilled on to it. As against this, a highly hydrophilic fleece will be supplied by simple capillarity, by immersing one of the edges thereof in the water. After this, the capillarity, even when this is small, of the fleeces, and the forces of gravity, will ensure spreading, retention and the flow of this water, during its downward travel through the fleeces. For a highly hydrophilic fleece, the throughput, per unit of immersed width, of the water thus pumped, decreases as the maximum height of the fleece above the water level concerned increases.
Thanks to the provisions according to the invention set out above, solar stills for producing fresh water are obtained which avoid several of the disadvantages of traditional solar units for distilling sea water. In effect, the mass of water heated by the sun is here reduced to a minimum value, as it is limited to the constantly-renewed small volume of water retained at each moment in the hydrophilic fleece. Under these conditions, the maximum temperature that this water can adopt is higher than that achieved in a conventional solar still, and the effect of this is to notably increase the saturated vapor pressure in the immediate vicinity of the hydrophilic fleece and, thus, the intensity of the evaporation achieved.
Additionally, as dwell time of the sea water in the installation is short (at the most, a few minutes), development of algae and mosses on this fleece is a priori excluded, even after several months of continuous operation. And this, all the more so as the moist fleece receives very little solar radiation, it being shaded by the dark-colored membrane.
According to one first particular embodiment of this still,
said membrane constitute the wall of an evaporation chamber that is relatively long, and installed underneath transparent sheltering means;
said transparent sheltering means is a closed space, relatively well sealed, the inner face of the wall thereof constituting said condensation surface;
a blower is associated with the apparatus for producing a flow of air in a closed circuit between the evaporation chamber and a condensation chamber, constituted by the volume of said space external of said evaporation chamber.
According to a second particular embodiment of this solar still:
the dark membrane constitute the outer wall of an evaporation chamber that is relatively long, installed underneath a transparent thermally-insulating cover;
the condensation surface is the inner impermeable side of the impermeable wall of a condensation chamber, arranged beneath the evaporation chamber and separated therefrom by a longitudinal common dividing wall, the two chambers communicating with each other by means of openings formed above transverse dividing walls separating the lower parts of their adjoining ends;
the external wall of said condensation chamber has a hydrophilic covering that is kept moist by any suitable means and at least partially exposed to the air;
a blower is associated with the apparatus for causing a closed-circuit flow of air from one chamber to another;
suitable means are associated with said blower to allow said two chambers to be inflated and to maintain a slight excess pressure inside them.
According to a third particular embodiment of this solar still:
the dark membrane constitute the outer wall of an evaporation chamber that is relatively long, installed beneath a transparent thermally-insulating cover;
the condensation surface is the inner side of the impermeable wall of a condensation chamber arranged, without a common wall, next to the evaporation chamber, the two chambers communicating with each other by means of two openings formed above transverse dividing walls separating the lower parts of their adjoining ends;
the outer wall of the condensation chamber includes a hydrophilic covering, which is protected from sun and kept moist by any suitable means and is at least partially exposed to the air;
a blower is associated with the apparatus for causing a closed-circuit flow of air inside the two chambers;
suitable means are associated with said blower to allow said two chambers to be inflated and to maintain a slight excess pressure inside them.
In the three particular embodiments of the invention, presented above, the closed-circuit air circulation inside the evaporation and condensation chambers considerably improves the conditions of transport of the water vapor, produced in the evaporation chamber, towards the condensation surface. In the first particular embodiment described, the condensed water droplets which fall from the roof of the closed space are no longer lost as they encounter the impermeable membrane before arriving at the fresh water collecting means provided for this purpose. Additionally, in the two other embodiments of the invention described above, we should stress the value of the presence of a transparent thermally-insulating cover installed above the portion of the evaporation chamber wall exposed to the sun. This cover prevents the formation of any droplets of condensed water and setting up a screen to the solar radiation, thereby allowing this portion of wall to receive at each moment a larger amount of heat and to consequently increase the intensity of evaporation of the water contained in the hydrophilic fleece. To this increased amount of heat, one can add a lower temperature of the condensation surface, which here depends on the temperature of the moist covering, exposed to the air, of this surface, and consequently on the dew point temperature and relative humidity of the outside air. By way of example, this transparent cover will be a thin sheet of plastic, with edges welded to the dark-colored membrane, thus entrapping a dry air layer of a few centimeters thick, or, yet again, a relatively thick plastic sheet having low thermal conductivity.
It will additionally be noted that a part of the condensation of vapor, achieved in the condensation chamber of the second embodiment of the invention, is produced with recovery of the latent heat on the dividing wall separating this chamber from the evaporation chamber; this enables this second embodiment of the invention to benefit from a slightly higher yield per unit. To finish on this point, it will be recognised that the second and third embodiments of the invention can be readily installed at any place (on the ground, on a terrace or a stretch of water) and that their ability to withstand strong winds is excellent thanks to the relatively high tension in the outer wall of the evaporation and condensation chambers which are maintained under slightly excess pressure.
A further advantage of the particular device for retaining the water to be distilled, which includes a solar still according to the invention, concerns the brine. In effect, it can be noted that, as soon as the throughput of sea water, distributed by the hydrophilic fleece over the impermeable membrane heated by the sun, is sufficiently high, no salt deposit can form. This throughput varies with the maximum intensity of the solar radiation of the place. It is determined by the ratio between the throughput of sea water and brine, which should always be distinctly less than eight, and for example be four. These figures result from two values, the salt concentration of the sea water which in general is about 30 grams/liter and the salt concentration threshold in brine, at which salt crystals can appear, which is about 240 grams/liter. If these conditions are respected, all the salt contained in the sea water circulating in the hydrophilic fleece is evacuated into the brine recovered, and no salt deposit can form in this fleece.
According to a general formulation of the optimum shape for the implementation of the present invention, a solar still for producing fresh water is characterised in that:
it comprises an evaporation chamber, a first condensation chamber and a second condensation chamber;
the evaporation chamber has a flexible wall, formed by an external impermeable membrane of a dark color, provided with an internal hydrophilic cover, this chamber being relatively long, exposed to the sun and installed beneath a transparent thermally-insulating cover;
a conduit supplied with water to be distilled is adapted to moisten, by capillarity and gravity, the hydrophilic fleeces that line the evaporation chamber;
the first condensation chamber has flexible walls formed by impermeable membranes and at least one of said membranes, provided with an outer hydrophilic coating supplied with water by capillarity and gravity, constituting common dividing wall separating the first condensation chamber from said evaporation chamber;
the downstream end of said evaporation chamber communicates with the inlet of said first condensation chamber by means of an opening adapted to prevent any passage of water to be distilled;
the downstream end of the said first condensation chamber discharges into the second condensation chamber, the outer wall of this second chamber being provided with a hydrophilic cover exposed to the air, protected from the sun and kept moist by a permanent supply of water;
the second condensation chamber communicates with the evaporation chamber through one opening adapted to prevent any passage of the water to be distilled;
an electric blower is associated with the apparatus to produce a closed-circuit flow of air inside the evaporation chamber and then inside the first condensation chamber and second condensation chamber;
suitable means are associated with said blower to allow the three chambers to be inflated and to maintain a slight excess pressure inside them;
a pipe for brine removal terminates at a low point of said evaporation chamber;
a pipe for removing distilled fresh water terminates at a low point in said second condensation chamber.
According to a supplementary characteristic, the geometric shape of the first condensation chamber and of the evaporation chamber depend on each other and are adapted to minimise heat exchange between this first chamber and the outside, and to maximise heat exchange between these two chambers.
According to a further characteristic, the first and second condensation chambers communicate with each other via links adapted to maximise heat exchange between the air flow leaving the first chamber and the inner side of the outer wall of the second condensation chamber.
According to a first particular optimum embodiment of solar still for producing fresh water according to the invention, the still takes the form of an elongate cylinder having a circular cross section, inside which the three chambers are installed and the common dividing wall separating the evaporation chamber from the first condensation chamber forms two planes arranged in a V-shape.
According to a second particular optimum embodiment, the solar still for producing fresh water is a type of large pneumatic mattress formed from a plurality of distillation cells;
each distillation cell includes, arranged in series, an evaporation chamber and two condensation chambers, a first one and, respectively, a second one;
the second condensation chamber of one cell being the preliminary chamber for the evaporation chamber of the following cell, when the cells are arranged in series;
a single blower, installed externally of the mattress formed by the distillation cells, so as to cause the air to circulate in a closed circuit, in all the chambers of the cells of the distillation apparatus;
symmetric oblique common dividing walls, fastened to the two faces of the apparatus, separating the evaporation chambers of the first condensation chambers and giving these chambers cross-sections in the form of narrow circular sectors;
a single thermally-insulating transparent cover covering the portion of the face of said apparatus exposed to the sun, formed by contiguous hot zones of the evaporation chambers
Thanks to these novel arrangements, these solar stills for producing fresh water, improved in accordance with the optimum embodiments of the invention, are suitable for distilling sea water and the majority of natural non-potable waters, with a particularly high yield.
This is firstly due to the particular characteristics of the invention already presented, which will not be commented on again. As against this, we should note what is stated below.
Inside the evaporation chamber, the temperature of the moist hydrophilic covering of the hot zone exposed to the sun is relatively high and clearly greater than that of the covering of the common dividing walls which are in the shade. The temperature of the hot moist air leaving the evaporation chamber in order to then penetrate into the first condensation chamber is comprised between these two temperatures.
Thanks to the particular cross-sectional shape of the common dividing walls separating the evaporation chambers from the first condensation chambers of the various distillation cells described above, the relative surface area of these dividing walls with respect to the surface area of the dark-colored walls exposed to the sun is relatively large. And, in the case of a solar still comprising several distillation cells, the total surface area of the common dividing walls forming the less-hot walls of the evaporation chamber can even be equal to several times that of the relatively hot zone exposed to the sun. Under these conditions, heat exchanges between the two chambers concerned are significant and the condensation of vapor is considerably favoured, as will be explained in detail below.
In the case of a still having a first condensation chamber that is thermally insulated with respect to the outside, the common dividing walls which separate this chamber from the evaporation chamber form a set of relatively cold zones that the hot moist air flow leaving the evaporation chamber encounters. Immediately, significant condensation of the water vapor transported by this air flow occurs, by diffusion on these dividing walls having a relatively large total surface area. The extent of this condensation decreases progressively as the current of air, which is less and less hot and moist, advances into the first condensation chamber. Throughout the passage through this first chamber, a significant proportion of the latent heat of condensation of the vapor is immediately recovered by the impermeable face of these dividing walls, and is continually recycled in order to participate in the evaporation of the water distributed by the hydrophilic fleece covering the outer face of these tubes. It will be noted that an identical, or even better, result is obtained when the first condensation chamber is completely surrounded by the evaporation chamber.
The wall of the second condensation chamber is relatively cold, and as the temperature of this wall constantly tends towards that of the dew point of the ambient air (that is particularly low in desert regions) due to the presence, around the second chamber, of an outer moist covering that is exposed to the air and sheltered from the sun. Additionally, the links between the first and second condensation chamber are arranged so that the flows of air leaving the first condensation chamber achieve the best sweeping of the internal face of the outer wall of this second chamber. For each design of the still, the relative length of this second condensation chamber is determined following routine tests, in order to optimize the temperature of the air flow leaving therefrom.
The passage through the second condensation chamber constitutes a last step in the production of fresh water during a cycle of circulation of the air inside a distillation still according to the invention. Under these conditions, the air which penetrates into the evaporation chamber of a distillation cell is relatively dry as it is relatively cold. And this, as will be seen later, particularly favours the effective performance of the dual function this air is to perform inside the evaporation chamber, specifically (1) to become progressively heated up and thus to become charged with a maximum amount of water vapor during its passage and (2) to allow effective recycling of the latent heat of the vapor which will then condense in the first condensation chamber.
In this respect, it will be noted that the presence of this second condensation chamber is a necessary condition for the possibility of more or less complete recycling of the latent heat of the vapor, performed through the common dividing walls separating the first condensation chamber from the evaporation chamber. In the second condensation chamber, the significant cooling of the air flow, achieved prior to its return into the evaporation chamber, is the phenomenon which constitutes the necessary condition for transfer of latent heat through the common dividing walls. In effect, thanks to this final cooling of the air flow, before its return into the evaporation chamber, two positive temperature differences, necessary for this transfer, are permanently established at a high level, at each side and over the full length of the common dividing walls. The first of these positive differences is set up between T1, the decreasing temperature of the air, which is initially hot and moist at the outlet from the evaporation chamber, during its travel through the tubes of the first condensation chamber and Tc, the temperature, which is also dropping, of the impermeable walls of the common dividing walls, from the inlet right up to the outlet of this first condensation chamber. Throughout the common dividing walls, from the inlet to the outlet of the first condensation chamber, the relation T1 greater than Tc holds. The second of these positive differences is the one between Tc, the increase in temperature of the moist hydrophilic covering over the whole length of the common dividing walls from the inlet right up to do the outlet of the evaporation chamber, and T2, the temperature, which is also increasing, of the moving air layers that sweep these moist coverings during their passage from the evaporation chamber. Over the whole length of the common dividing walls, from the inlet up to the outlet of the evaporation chamber, the relation Tc greater than T2 holds. Consequently, this phenomenon is the actual one which establishes the necessary conditions for high-amplitude recycling of the latent heat of the transported vapor, during a period of circulation of the air in a distillation cell. The effect of this phenomenon is notably to increase the yield of the solar still for producing fresh water according to the invention, up to a value well above unity. It will be noted that this yield increases with the value of the ratio between total surface area of the common dividing walls on which the latent heat of condensation of vapor is recycled, and the surface area of the hot zone of the evaporation chamber.
From the above, it results that the various stages in the cycle of circulation of the air in the three successive chambers of a distillation cell of a solar still for producing fresh water according to the invention are optimized. This makes a considerable contribution to improving the yield of such a still.
Additionally, such an improved solar still can very readily be adapted to external installation and implementation conditions imposed on it (whether on the ground or on a stretch of water) and it is able to withstand strong winds. The reason for this is to be seen in the tension in the chamber walls and their suitably-inflated insulation covering which is greater than the pressure exerted by the strongest normal winds while a second reason lies in the effective means used for securing the apparatus which are simple to install and implement. These advantages are particularly apparent for a solar still having multiple distillation cells, since, for a large absorbing surface area, volume occupied is reduced, as is relative height.